streams issue

build.sh

#!/bin/sh

c++ main.cc -o main -std=c++20 && ./main

main.cc

#include <sstream>
#include <iostream>
#include <locale>
#include <codecvt>

template< typename stream >
void test(stream&& ss)
{
    // stream ss("abc");
    for(int i = 0; i < 4; ++i)
    {
        std::cout << (int)ss.get() << ' ';
    }
    std::cout << std::endl;

    ss.clear();
    ss.seekg(0, ss.beg);
    for(int i = 0; i < 4; ++i)
    {
        std::cout << (int)ss.get() << ' ';
    }
    std::cout << std::endl;
}

int main()
{
    test< std::istringstream >(std::istringstream{"abc"});
    std::cout << "---" << std::endl;

    test< std::wistringstream >(std::wistringstream{L"abc"});
    std::cout << "---" << std::endl;

    test< std::wistream >(std::wistringstream{L"abc"});
    std::cout << "---" << std::endl;

    {
        std::stringbuf buf;
        buf.str("abc");
        std::wbuffer_convert< std::codecvt_utf8< wchar_t >> conv(&buf);
        test< std::wistream >(std::wistream(&conv));
    }

    return 0;
}

output

97 98 99 -1 
97 98 99 -1 
---
97 98 99 -1 
97 98 99 -1 
---
97 98 99 -1 
97 98 99 -1 
---
97 98 99 -1 
-1 -1 -1 -1